1. Field of the Invention
The present invention relates to a thermoelectric material and a method of manufacturing the material, and more specifically, to a thermoelectric material containing a half-Heusler compound as a major component, and a method of manufacturing the material.
2. Description of the Related Art
Thermoelectric conversion refers to direct conversion of electric energies into thermal energies for cooling or heating with the use of the Seebeck effect or the Peltier effect, and vice versa. The thermoelectric conversion has the following characteristics:    (1) Extra waste materials are not discharged during the energy conversion;    (2) Exhaust heat can be effectively used;    (3) Electric power generation can be continued until constituent materials are deteriorated; and    (4) A movable device such as a motor or turbine is not necessary, and hence maintenance is not needed. Therefore, the thermoelectric conversion attracts attention as a technique for highly-efficient utilization of energy.
As an index for evaluating the property of a material that can convert thermal energies and electric energies into one another, that is, a thermoelectric material, a figure of merit Z (=S2σ/κ, where S, σ and κ are Seebeck coefficient, electric conductivity and thermal conductivity, respectively), or a dimensionless figure of merit ZT given as a product of the figure of merit Z by the absolute temperature T at which the figure of merit Z is indicated, is generally used. Alternatively, as an index for evaluating the property of a thermoelectric material, a power factor PF (=S2σ) is sometimes used.
The Seebeck coefficient represents the magnitude of an electromotive force generated by a temperature difference of 1K. Thermoelectric materials respectively have the Seebeck coefficients inherent thereto, and are roughly divided into two groups, one of which has positive Seebeck coefficients (p-type) and the other of which has negative Seebeck coefficients (n-type).
Thermoelectric materials are typically used in a state where a p-type thermoelectric material and an n-type thermoelectric material are joined together. Such a joint pair is generally called a “thermoelectric device”. The figure of merit of a thermoelectric device is dependent on the figure of merit Zp of a p-type thermoelectric material, the figure of merit Zn of an n-type thermoelectric material and the shape of the p-type and the n-type thermoelectric materials. It is known that, when the shape thereof is optimized, the figure of merit of the thermoelectric device becomes larger as Zp and/or Zn are larger. Accordingly, in order to obtain a thermoelectric device having a high figure of merit, it is important that thermoelectric materials having high figures of merit Zp and Zn are used.
As such thermoelectric materials, the followings are known:    (1) Compound semiconductors such as Bi—Te, Pb—Te and Si—Ge based alloys;    (2) Skutterudite compounds such as Zn—Sb, Co—Sb and Fe—Sb based alloys; and    (3) Half-Heusler compounds such as TiNiSn.
Among them, the Bi—Te and Pb—Te based compound semiconductors exhibit high ZT in the low temperature range; however, there is a problem in that the semiconductors cannot be used in the intermediate to high temperature range, and that the semiconductors contain large amounts of elements having large environmental burdens, such as Pb, Te and Sb. Further, the Ge—Si based compound semiconductors contain large amounts of expensive Ge.
The skutterudite compounds are p-type thermoelectric materials exhibiting relatively high thermoelectric properties in the low to intermediate temperature range. It is known that certain skutterudite compounds have ZT greater than 1 at 527° C. (800 K). For example, because the temperature of automotive exhaust gas is approximately 800 K, it is expected that, when a thermoelectric device using such skutterudite compounds is employed, an exhaust heat recovery system with high-efficiency could be obtained. However, there is a problem in that many of the skutterudite compounds exhibiting high thermoelectric properties in the low to intermediate temperature range contain large amounts of elements having large environmental burdens such as Sb.
Contrary to this, the TiNiSn or ZrNiSn based half-Heusler compounds have characteristics that the compounds exhibit high thermoelectric properties in the low to intermediate temperature range and that the compounds do not contain any element having environmental burdens. Herein, “half-Heusler compounds” mean a series of compounds having the structure in which half the atoms at Cu sites of Heusler alloy Cu2AlMn are deficient. Although both the TiNiSn compounds and the ZrNiSn compounds essentially have high power factors, there is a problem in that those compounds have limits in reachable figures of merit due to their high thermal conductivities.
In order to solve the aforementioned problem, various proposals have been conventionally made. For example, Patent Document 1 discloses a thermoelectric material having a composition represented by (Ti0.98Zr0.01Hf0.01)NiSn.
In the document, the following descriptions are made:    (1) When all of Ti, Zr and Hf are contained in the A sites of MNiSn, which is a type of the half-Heusler compounds represented by the general formula ABX, the thermal conductivity thereof can be reduced;    (2) Lattice thermal conductivity of the material having the composition of (Ti0.98Zr0.01Hf0.01)NiSn at 300 K is 3.71 W/mK, and the dimensionless figure of merit thereof at 300 K, is 0.05; and    (3) The lattice thermal conductivity of TiNiSn at 300 K is 9.75 W/mK.
Patent Document 2 discloses a thermoelectric material having the composition represented by (Ti0.25Zr0.45Hf0.30)33Ni34(Sn0.996Sb0.004)33, in which an existence ratio of Ti—X (where X is at least one selected from Sn and Sb) phase is 8.5%.
In the document, the following descriptions are made:    (1) When the base alloy is treated at 1200° C., the existence ratio of Ti—X phase is decreased and the thermoelectric property thereof is improved; and    (2) When part of Ti is substituted with Zr or Hf, the thermal conductivity can be reduced.
Patent Documents 3 and 4 disclose a thermoelectric material having the composition represented by (Ti0.3Zr0.35Hf0.35)NiSn.
In the documents, the following description is made: when two or more elements selected from the group consisting of Ti, Zr and Hf are used at M sites of the half-Heusler compounds represented by the general formula Mαβ, the thermal conductivity can be greatly reduced.
Non-patent Document 1 discloses a TiNiSn single crystal synthesized by the Sn-flux method. In the document, the following description is made: the dimensionless figure of merit ZT of the TiNiSn single crystal at 300 K is 0.09. Further, Patent Document 5 discloses a TiNiSn single crystal synthesized by the unidirectional solidification method.
Patent Document 6 discloses a thermoelectric material having the composition represented by Ti0.95Hf0.05NiSn0.99Sb0.01.
In the document, the following description is made: the power factor of Ti0.95Hf0.05NiSn0.99Sb0.01 at 700 K is 4.1 mW/mK2, whereas that of TiNiSn at 700 K is 1.8 mW/mK2.
In Non-patent Documents 2 and 3, the following description is made: the thermal conductivity of TiNiSn at room temperature is 7 to 8 W/mK.
Non-patent Document 4 discloses TiNi1.5Sn obtained by melting a raw material in a high-frequency induction melting furnace to be cast, and then by remelting the resultant ingot to be rapidly solidified. In the document, the following description is made: a mixed phase including the half-Heusler phase and the full-Heusler phase can be obtained by such a method.
Further, in Non-patent Document 5, the following description is made: reduction in the thermal conductivity of ZrNiSn can be realized by substituting some of the Ni sites thereof with Pd. Further, Non-patent Document 6 reports an effect of reducing the thermal conductivity by substituting part of the Ni sites with Pt.    [Patent Document 1] Japanese Patent Application Publication No. 2004-356607    [Patent Document 2] Japanese Patent Application Publication No. 2006-269731    [Patent Document 3] Japanese Patent Application Publication No. 2005-286228    [Patent Document 4] Japanese Patent Application Publication No. 2007-158191    [Patent Document 5] Japanese Patent Application Publication No. 2006-228912    [Patent Document 6] Japanese Patent Application Publication No. 2005-019713    [Non-patent Document 1] W. Kafer et al., Inst. Phys. Conf. Ser. No. 152, 185(1997)    [Non-patent Document 2] S. Bhattacharya et al., Phys. Rev. B77, 184203(2008)    [Non-patent Document 3] S. W. Kim et al., Sci. and Tech. of Adv. Matter. 5, 485(2004)    [Non-patent Document 4] T. Morimura et al., J. Alloys and Compounds 416, 155(2006)    [Non-patent Document 5] Q. Shen et al., Appl. Phys Letter 79 (2001) 4165-4167    [Non-patent Document 6] S. Culp et al., Proceeding of ICT 2005
The TiNiSn based compound has a relatively high power factor PF; however, there is a problem in that the thermal conductivity κ is high. On the other hand, the method of substituting some of the Ti sites of the TiNiSn based compound with a heavy element such as Zr and Hf, is effective as a way to reduce the thermal conductivity κ of the TiNiSn based compound. However, heavy elements such as Zr and Hf (in particular, Hf) are expensive in comparison with Ti, causing a problem in terms of cost.
Further, when synthesizing the TiNiSn based compound, a secondary phase such as the full-Heusler phase or Ti6Sn5 is sometimes generated depending on the composition and the manufacturing conditions. If the secondary phase is a metallic phase, precipitation of the secondary phase may cause the thermoelectric property thereof to be deteriorated.
The same is true with the ZrNiSn based compound.